Abstract:

The increase in atmospheric greenhouse gas concentrations from anthropogenic activities is the major driver of recent global climate change1. The stimulation of plant photosynthesis due to rising atmospheric carbon dioxide concentrations ([CO2]) is widely assumed to increase the net primary productivity (NPP) of C3 plants—the CO2 fertilization effect (CFE). However, the magnitude and persistence of the CFE under future climates, including more frequent weather extremes, are controversial. Here we use data from 16 years of temperate grassland grown under ‘free-air carbon dioxide enrichment’ conditions to show that the CFE on above-ground biomass is strongest under local average environmental conditions. The observed CFE was reduced or disappeared under wetter, drier and/or hotter conditions when the forcing variable exceeded its intermediate regime. This is in contrast to predictions of an increased CO2 fertilization effect under drier and warmer conditions. Such extreme weather conditions are projected to occur more intensely and frequently under future climate scenarios. Consequently, current biogeochemical models might overestimate the future NPP sink capacity of temperate C3 grasslands and hence underestimate future atmospheric [CO2] increase.

Abstract:

The sampling design in the all six Gi-FACE rings is standardised. Subplots 1-3 are used for weekly measurements of greenhouse gas fluxes and biannual biomass harvest. Subplots 4-25 are used for biannual biomass harvests and cover more or less the two species inventory subplots for biannual vegetations surveys. Subplots 24 and 25 have been used in 2013 for a 15N-13C Tracing study and in Subplot 25 the topsoil (7.5 cm) was used in 2014 for an incubation study and the soil was replaced by soil from outside the rings. Small circles within the ring area indicate Licor soil colomne for soil respiration measurements. The yellow rectangles mark the area where soil profiles have been opened in 2015 to install new soil moisture and temperature sensors. "Needles" and asterix mark positions where soil samples have been taken.

Abstract:

An increase in mean soil surface temperature has been observed over the last century and it is predicted to further increase in the future. The effect of increased temperature on ecosystem carbon fluxes in a permanent temperate grassland, was studied in a long term (6 years) field experiment, using multiple temperature increments induced by IR-lamps. Ecosystem respiration (R-eco) and net ecosystem exchange (NEE) were measured, and modelled by a modified Lloyd and Taylor model including a soil moisture component for R-eco (average R2 of 0.78) and inclusion of a photosynthetic component based on temperature and radiation for NEE (R2=0.65). Modelled NEE values ranged between 2.3 and 5.3 kg CO2 m-2 year-1, depending on treatment. An increase of 2 or 3°C led to increased carbon losses, lowering the carbon storage potential by around 4 tonnes of C ha-1 year-1. The majority of significant NEE differences were found during night-time compared to daytime. This suggests that during daytime the increased respiration could be offset by an increase in photosynthetic uptake. This was also supported by differences in ?13C and ?18O, indicating prolonged increased photosynthetic activity associated with the higher temperature treatments. However, this increase in photosynthesis was insufficient to counteract the 24hr increase in respiration, explaining the higher CO2 emissions due to elevated temperature.

Abstract:

Soil respiration of terrestrial ecosystems, a major component in the global carbon cycle is affected by elevated atmospheric CO2 concentrations. However, seasonal differences of feedback effects of elevated CO2 have rarely been studied. At the Gießen Free-Air CO2 Enrichment (GiFACE) site, the effects of +20% above ambient CO2 concentration have been investigated since 1998 in a temperate grassland ecosystem. We defined five distinct annual seasons, with respect to management practices and phenological cycles. For a period of 3 years (2008–2010), weekly measurements of soil respiration were carried out with a survey chamber on vegetation-free subplots. The results revealed a pronounced and repeated increase of soil respiration under elevated CO2 during late autumn and winter dormancy. Increased CO2 losses during the autumn season (September–October) were 15.7% higher and during the winter season (November–March) were 17.4% higher compared to respiration from ambient CO2 plots.
However, during spring time and summer, which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was observed at the GiFACE site under elevated CO2. This suggests (1) that soil respiration measurements, carried out only during the growing season under elevated CO2 may underestimate the true soil-respiratory CO2 loss (i.e. overestimate the C sequestered), and (2) that additional C assimilated by plants during the growing season and transferred below-ground will quickly be lost via enhanced heterotrophic respiration outside the main growing season.

Abstract:

A major issue of our today's research is to help meeting the challenges of future food security. One task is to assess and develop crop management strategies adapted to predicted future climatic conditions. Yet, both the variability of environmental conditions and the uncertainty of climate projections as well as the orchestra of multiple plant responses and their interaction with the environment make it difficult to predict plant behavior in the field. Recent studies have demonstrated the usefulness of classical crop models as a tool to investigate crop productivity under predicted climate conditions. These models use data on plant architecture only to a limited extent as they usually follow a systems approach by focusing on processes for predicting dry matter production. However, plant architecture is a major determinant of the crops’ resource use efficiency. Moreover, plants show time dependent structural changes as they grow and develop, and these processes are affected by various environmental factors and stresses. Virtual plant models consider both the three-dimensional plant architecture and concepts of plant physiology. Here, we outline the way in which virtual plant modelling can further improve our understanding on the impact of climate change on food production. Greenhouse and growth chamber experiments may serve as data sources for model parameterization, in particular of response functions with respect to environmental stimuli. Data from field experiments in free air carbon enrichment (FACE) facilities, such as those obtained in the new Geisenheim FACE for special crops, may be used to evaluate virtual plant models with respect to future climatic conditions. A combination of field data and virtual plant model simulations may then allow us to assess the specific role of plant architecture in resource use efficiency and help to develop advanced strategies for future crop production.

Abstract:

Future vegetable crop production might be affected by Climate Change with the projected increase in atmospheric CO2 concentration and changes in precipitation pattern. Elevated CO2 as one main component of photosynthesis is assumed to increase the yield of many crops while alleviating negative effects of drought stress and, thereby, increasing the water use efficiency. However, the responses of field grown vegetable crops to elevated CO2 are still unknown as previous findings are mainly derived from experiments conducted under controlled environments. In addition, the field vegetable production is characterized by several sets per season, varying growth conditions during the production periods and a high water demand usually provided by an irrigation system. Moreover, vegetable crops differ in the harvest organs, e.g. fruits, root tuber, bulbs or leaves, which are predominantly harvested in an early development stage. These vegetable-specific aspects have not been considered in past Free Air Carbon Dioxide Enrichment (FACE) experiments. Therefore, we aim at analyzing the short and long impacts of elevated CO2 with limited water supply on field vegetable crop productivity for three different crops (Cucumis sativus L., Raphanus sativus var. sativius L., Spinacia oleracea L.). Here, we present the methodological framework. Experiments will be conducted in the newly established FACE facility for vegetable crops at Geisenheim University, Germany. The facility is designed to raise the ambient CO2 concentration at the experimental field site by about 20% to approximately 480 ppm and to regulate the water supply with a drip irrigation system, resulting in a split plot design with three replications. In each replication an annual crop rotation with several production cycles of the three different vegetable crops are realized. Measurements of the CO2 and H2O gas exchange on leaf level as well as non-destructive and destructive recordings of plant growth and development are planned.

Abstract:

The rising concentration of carbon dioxide in the atmosphere ([CO2]) has a direct effect on terrestrial vegetation
through shifts in the rates of photosynthetic carbon uptake and transpirational water-loss. Free Air CO2 Enrichment (FACE)
experiments aim to predict the likely responses of plants to increased [CO2] under normal climatic conditions. The Giessen
FACE system operates a lower [CO2] enrichment regime (480 mmol mol–1) than standard FACE (550–600 mmol mol–1),
permitting the analysis of a mixed species temperate meadow under a [CO2] level equivalent to that predicted in 25–30 years.
We analysed the physiological and morphological responses of six species to investigate the effect of moderate [CO2] on
spring biomass production. Carbon dioxide enrichment stimulated leaf photosynthetic rates and supressed respiration,
contributing to enhanced net assimilation and a 23% increase in biomass. The capacity for photosynthetic assimilation was
unaffected by [CO2] enrichment, with no downregulation of rates of carboxylation of Rubisco or regeneration of ribulose-
1,5-bisphosphate. Foliar N content was also not influenced by increased [CO2]. Enhanced [CO2] reduced stomatal size, but
stomatal density and leaf area index remained constant, suggesting that the effect on gas exchange was minimal.